One-dimensional group III nitride nanostructures, such as aluminum gallium nitride (AlGaN) or AlN nanostructures, have recently been investigated due to their unique physical properties for applications ranging from electronics to biomedical applications. For example, AlN nanowires have attracted particular attention due to their unique field emission properties, their electrical transport properties, as well as mechanical and piezoelectric properties. AlN nanowires have substantially different properties when compared to their bulk material counterparts, offering a potential for novel technological applications. Among the group III nitrides, AlN is a promising candidate for potential applications in nano-electronics due to its large energy band gap of 6.2 eV (i.e., the highest band gap among the group III nitrides) and low values of electron affinity. Other desirable properties of AlN include: high thermal and chemical stability, high thermal conductivity, high melting temperature, as well as low thermal expansion coefficients and good mechanical strength.
A wide range of AlN-based one dimensional (1D) nanostructures have been successfully synthesized recently, such as nano-whiskers, nanowires, nano-tubes, nano-needles, nano-cones, nano-fibers, and many others. There have been a number of different fabrication techniques used to synthesize AlN nanowires, including: a template-confined method, a direct current (DC) arc discharge method, a catalyst-assisted vapor-liquid-solid (VLS) growth method, as well as a catalyst-free vapor-solid (VS) growth method.
The template-confined method achieves formation of AlN nanowires through the use of template growth. Generally, the template-confined synthesis method is simple and provides a convenient way for bulk fabrication of AlN nanowires with uniform and controllable geometry. Materials, such as carbon nanotubes and anodic porous aluminum oxide, have been used to confine the formation of AlN nanowire within the nanometer scale. For example, AlN nanowires with controlled diameter have been successfully synthesized. These samples were fabricated through the reaction of carbon nanotubes, Al, and Al2O3 in a flowing NH3 atmosphere. In this approach, Al powder of 200 mesh and Al2O3 powder of the same scale were mixed together in a 1:1 weight proportion. These, along with carbon nanotubes, served as the raw materials for producing the AlN nanowires. Anodic Aluminum Oxide (AAO) has also been explored as a template material for growth of AlN nanowires. Using an AAO template, hexagonal AlN nanowire arrays have been fabricated through the direct reaction of metal Al vapor with NH3/N2 at a temperature of 1100 Celsius under the confinement of an anodic porous alumina template. The synthesis mechanism was understood as the space-limited nucleation followed by the growth along the template channels.
The DC arc discharge method is a common technique used to synthesize 1D semiconductor nanostructures. One of the most notable advantages of this technique is that it enables fabrication of nanostructures with high crystal quality due to the high growth temperature and temperature gradient used. The successful synthesis of 1D AlN nanowires by the DC arc plasma method was recently reported. First, aluminum was melted in a N2—Ar ambient through arc plasma discharge. Subsequently, under highly non-equilibrium conditions, the molten Al reacted with N2 to produce crystalline cubic AlN (c-AlN) nanowires and nanoparticles. With an increased arc current, a simultaneous precipitation of c-AlN along with h-AlN phase was obtained.
The catalyst-assisted VLS growth method typically requires a metal for the formation of nanowires. The nanowire growth is characterized by a liquid-solid interface, where the synthesis of nanowire is accomplished through a liquid droplet. The growth sequence of the nanowires based on the VLS mechanism can be divided into three stages: (1) formation of the liquid-forming catalyst, (2) nucleation and growth of alloy droplets, and (3) growth of the nanowire from the liquid droplets due to super-saturation. The droplet is typically located at the end of the growing nanowire and serves as a region for the incorporation of aluminum and nitrogen for subsequent formation of the nanowire.
Finally, the catalyst-free VS growth method assumes the growth without the presence of any catalyst. This method is typically less complicated than the other growth approaches. In general, in a VS mechanism, the anisotropic growth of semiconductor nanowires is driven by several factors, such as the difference in the growth rates of various crystal facets. The VS growth process involves the use of aluminum and nitrogen as vapor precursors. The precursors are condensed onto a substrate. Due to the super-saturation of the vapor in gas phase, nucleation of the growth species on the substrate occurs. Eventually, with proper control of the experimental conditions, the desired semi-conductor nanowires can be successfully obtained. By employing a simple physical vapor deposition, highly ordered AlN nanowires have been successfully fabricated on a sapphire substrate. The resultant well-aligned nanowires had a uniform distribution in both their diameters and lengths. The nanowire diameter appeared to narrow toward the tip of the nanowire. The process of growth followed the steps of: producing an aluminum vapor reacting with NH3 to form AlN on a sapphire substrate. The art of controlling desired AlN nanowire structure is difficult through a regular VS mechanism.